Faster Exact and Parameterized Algorithm for Feedback Vertex Set in Bipartite Tournaments Mithilesh Kumar1 and Daniel Lokshtanov2 1 Department of Informatics, University of Bergen Norway [email protected] 2 Department of Informatics, University of Bergen Norway [email protected] Abstract A bipartite tournament is a directed graph T := (A ∪ B, E) such that every pair of vertices (a, b), a ∈ A, b ∈ B are connected by an arc, and no arc connects two vertices of A or two vertices of B.A feedback vertex set is a set S of vertices in T such that T − S is acyclic. In this article we consider the Feedback Vertex Set problem in bipartite tournaments. Here the input is a bipartite tournament T on n vertices together with an integer k, and the task is to determine whether T has a feedback vertex set of size at most k. We give a new algorithm for Feedback Vertex Set in Bipartite Tournaments. The running time of our algorithm is upper-bounded by O(1.6181k + nO(1)), improving over the previously best known algorithm with running time 2kkO(1) + nO(1) [Hsiao, ISAAC 2011]. As a by-product, we also obtain the fastest currently known exact exponential-time algorithm for the problem, with running time O(1.3820n). 1998 ACM Subject Classification F.2.2 Nonnumerical Algorithms and Problems Keywords and phrases Parameterized algorithms, Exact algorithms, Feedback vertex set, Tour- naments, Bipartite tournaments Digital Object Identifier 10.4230/LIPIcs.xxx.yyy.p 1 Introduction A feedback vertex set in a graph G is a vertex set whose removal makes the graph acyclic. The Feedback Vertex Set problem is a well-studied graph problem where input is a graph G (directed or undirected) and the task is to find a smallest possible feedback vertex set. Finding such an optimal feedback vertex set turns out to be NP-complete [22], indeed the problem is one of the very first to be shown NP-complete in the influential paper of Karp [26]. Since, polynomial time algorithms are highly unlikely, Feedback Vertex Set on general directed and undirected graphs has been extensively studied from the perspective of approximation algorithms [2, 15], parameterized algorithms [6, 10, 27], exact exponential-time algorithms [29, 34] as well as graph theory [14, 30]. This paper belongs to a long line of work studying the complexity of Feedback Vertex Set on restricted classes of graphs. On one hand Feedback Vertex Set remains NP- complete on tournaments and bipartite tournaments [5], planar undirected graphs [22], planar directed graphs with in-degree and out-degree at most 3 [22] as well as directed graphs with in-degree and out-degree at most 2 [22]. On the other hand the problem is polynomial time solvable on undirected graphs of maximum degree 3 [33], chordal graphs [16] and weakly © Mithilesh Kumar and Daniel Lokshtanov; licensed under Creative Commons License CC-BY Conference title on which this volume is based on. Editors: Billy Editor and Bill Editors; pp. 1–17 Leibniz International Proceedings in Informatics Schloss Dagstuhl – Leibniz-Zentrum für Informatik, Dagstuhl Publishing, Germany 2 Faster Exact and Parameterized Algorithm for Feedback Vertex Set in Tournaments chordal graphs [20], indeed on any class of graphs with polynomially many potential maximal cliques [20]. Being a problem of fundamental importance, Feedback Vertex Set has been approached algorithmically even on the classes of graphs where it remains NP-complete. For example the problem admits (efficient) polynomial time approximation schemes [8, 12, 18], sub-exponential time parameterized algorithms [11] and linear kernels [19] on classes of graphs excluding a fixed graph H as a minor. In this paper we study the problem on bipartite tournaments. A tournament is a subclass of directed graphs where every pair of vertices are connected by an arc. A bipartite tournament is a directed graph where the vertices are partitioned into two sets A and B, there is an arc connecting every vertex in A with every vertex in B, and there are no edges between vertices of A and vertices of B. Tournaments arise naturally from round-robin competitions whereas bipartite tournaments model a two-team competition in which every player in one team plays against every player of the other team. Here arcs are drawn from the winning to the losing player, and often one seeks to rank the players from “best” to “worst” such that players that appear higher in the ranking beat all lower ranked players they played against. Such an absolute ranking possible only if there are no cycles in the tournament. The size of the smallest feedback vertex set then becomes a measure of how far the tournament is from admitting a consistent ranking. For this reason the structure of cycles and feedback vertex sets in (bipartite) tournaments has been studied both from the perspective of graph theory [3, 7, 21] and algorithms. For bipartite tournaments, finding a feedback vertex set reduces to hitting all cycles of length 4. For this reason the Feedback Vertex Set problem is more computationally tractable on bipartite tournaments than on general directed graphs. Specifically the best known approximation algorithm for Feedback Vertex Set on directed graphs has an approximation factor of O(log n · log log n) [15], and the problem does not admit a constant factor approximation assuming the Unique Games Conjecture [23]. On bipartite tournaments it is easy to obtain a 4-approximation (see Lemma 2). Further, an improved approximation algorithm with ratio 3.5 was obtained by Cai et al. [4]. Similarly, it was open for a long time whether Feedback Vertex Set on general directed graphs admits an FPT algorithm, that is an algorithm that determines whether there exists a solution of size at most k in time f(k)nO(1). In 2008, Chen et al. [6] gave an algorithm with running time O(4kkO(1)k!nm), and it is an outstanding open problem whether there exists an algorithm with running time 2O(k)nO(1). For bipartite tournaments, the realization that it is necessary and sufficient to hit all cycles of length 4 yields a simple 4knO(1) time parameterized algorithm: recursively branch on vertices of a cycle of length 4. Truß [32] gave an improved algorithm with running time 3.12knO(1), Sasatte [31] further improved the running time to 3knO(1), while Hsiao [25] gave an algorithm with running time 2knO(1). Prior to this work, this was the fastest known parameterized algorithm for Feedback Vertex Set on bipartite tournaments. Our main result is an algorithm with running time O(1.6181k + nO(1)). Using the recent black-box reduction from parameterized to exact exponential time algorithms of Fomin et al. [17] we also obtain an exponential-time algorithm running in O(1.3820n) time. Methods. Our algorithm is based on the recent parameterized algorithm with running time O(1.6181k + nO(1)) by the authors [28] for Feedback Vertex Set in tournaments. The main idea of this algorithm is that tournaments are very rigid. Given as input a tournament T , by obtaining a large set M of vertices that is disjoint from the feedback vertex set H sought for, we can get a rough sketch of the rigid structure of T − H. This structure is then very useful for recovering the solution H. Indeed, the only way that vertices that are “far M. Kumar and D. Lokshtanov 3 apart” in the approximate sketch of the structure of T − H can interact with each other is by being “in conflict”. Out of two vertices that are in conflict, one of them has to be deleted. Thus, dealing with conflicts can be done in a similar fashion as with edges in the Vertex Cover problem. For any vertex v appearing in at least two conflicts, branch into two sub-problems. In the first sub-problem v is deleted, in the second all vertices in conflict with v are deleted. If there are no conflicts it is sufficient to solve the Feedback Vertex Set problem “locally”. If every vertex appears in at most one conflict a divide and conquer approach can be taken. Because bipartite tournaments are also quite “rigid”, we expected that the same approach would easily give an algorithm for Feedback Vertex Set on bipartite tournaments with the same running time. Our expectations were both wrong and correct; indeed we do obtain an algorithm for Feedback Vertex Set on bipartite tournaments with the same template and the same running time as the algorithm for tournaments [28], yet the adaptation turned out to be anything but easy. Specifically, in virtually every step of the algorithm, the lack of a unique topological sort of acyclic bipartite tournaments presented significant challenges. The fact that these challenges still could be overcome by sub-exponential time cleaning procedures gives hope that the same template could be applicable in several situations where one seeks a “small” set of vertices or edges to delete in order to modify the input graph to a “rigid” structure; such as Cluster Vertex Deletion, Cograph Vertex Deletion and Feedback Vertex Set in the more general setting when the input graph is a multi-partite tournament [24]. Organization of the paper. In Section 2 we set up definitions and notation, and state a few useful preliminary results. The standard graph notation and parameterized complexity terminology is set up in the appendix. In Section 3 we define and prove some properties of M-sequence. In Section 4 we define and give an algorithm for Constrained Feedback Vertex Set problem. 2 Preliminaries In this paper, we work with graphs that do not contain any self loops.